The earlier 6000 Vortex Series Tunable Diode Laser, produced by New Focus Corp. of San Jose, Calif., employs a laser cavity based on the Littrnan-Metcalf design, which uses a diffraction grating at grazing incidence, together with a tuning reflector, to provide wavelength selectivity. Essential to the performance of tunable external-cavity diode lasers (ECDLs) is a high-quality anti-reflection (AR) coating on the front facet of the diode. The AR coating turns the diode into purely a gain element. A collimating lens directs the output of the diode across a diffraction grating at grazing incidence. The end mirror of the laser cavity reflects the first-order diffraction off the grating to provide feedback. Dispersion provided by the grating allows only one cavity mode to lase, resulting in a very narrow linewidth. The specular reflection or zero-order diffraction off the grating serves as the output beam of the laser.
The angle between the grating and the end mirror determines the lasing wavelength. Tuning is achieved by varying the angle using a piezoelectric actuator to rotate the end mirror. Continuous (mode-hop-free) tuning requires selecting an appropriate rotation point, also called a pivot point. Discontinuous tuning, characterized by period “mode-hops” results from two competing wavelength-selection constraints, the mirror-grating angle and the laser-cavity length. The laser-cavity length, L, defines a discrete set of possible wavelengths or modes, λN, that can lase, given by the equation L=NλN/2, (N=integer). The grating equation insists that mλ=Λ(sin θi+sin θd), where m stands for the grating diffraction orders. Λ refers to the groove spacing of the grating while θi and θd refer to the incident and diffracted angles of the laser beam. Rotation of the tuning reflector causes parameters in both equations to change. An appropriately selected point of rotation synchronizes the two, such that the cavity length remains the same number of half-wavelengths long as the tuning reflector is rotated. Thus mode-hop free tuning is achieved. When this condition is not met, the lasing wavelength will periodically hop from one mode to the next (e.g., from N to N+1).
U.S. Pat. No. 5,995,521, entitled External Cavity Laser Pivot Design, invented by Moore et al., discloses an external cavity diode laser (ECDL) with a pivot point design that is inexpensive and precise and that allows for simultaneous rotary and lineal motion. A housing for a tunable external cavity laser including a gain medium, a reflector, and a dispersive element is disclosed. The housing includes a base section, a motion section and an actuator. Glue is used to secure together certain non-movable parts. The gain medium and a diffractor are mounted to the base section such that a first energy beam emanating from the gain medium strikes the dispersive element and leaves the dispersive element as a second beam at an angle with respect to the first beam which depends on wavelength. A reflector is conditioned on the motion section to reflect the second beam to the gain medium via the dispersive element. The motion section is supported by the base about a torsional member, which defines a rotational axis for minor movement. The actuator is in selectively engagable abutment against the motion section to move the reflector with respect to the dispersive element about the torsional member to tune the external cavity laser.
While this arrangement generally has been successful, there has been a need for an improved laser tuning mechanism and for an associated tunable external cavity laser system. More particularly, there has been a need for an improved tuning mechanism for continuous tuning without mode-hop, which can provide fine increment control of wavelength, and that is simple and cost-effective in design. For example, there has been a need for a rotational axis which is subject to better control than that defined by the torsional member. Moreover, there has been a need for an improved design that provides stability of laser wavelength tuning even in harsh environments such as the ones with high or low temperatures, pressure variation or humidity change. There also has been a need for such a laser tuning mechanism and associated external cavity laser system that is suitable for the vacuum environment for outer space. It will be appreciated that the use of glue may be unsuitable in a vacuum environment due to outgassing. The present invention meets these needs.